Method and apparatus for increasing the shear strength of a construction structure

ABSTRACT

In order to strengthen an elongated or a two-dimensional bearing construction structure (1) against shear forces, at least one pre-stressing means (11) is mounted in a slack or pre-stressed manner on or in the structure cross-section (4) to generate a pre-stressing force directed transversely to the length or to the two-dimensional surface of the structure when this structure is shear-loaded.

The present invention concerns a method for increasing the shearstrength of an elongated or a substantially two-dimensional constructionstructure serving as a support; further a method for increasing theshear strength of a construction structure with an elongated, laminarreinforcement affixed from the outside to the structure to reinforce it;further a method for increasing the shear strength of an elongated ortwo-dimensional concrete construction structure reinforced on its insidewith steel means; applicability of the above methods; apparatus toincrease the shear strength of an elongated or substantiallytwo-dimensional bearing construction structure, a bearing constructionstructure such as of steel reinforced concrete; with apparatus and aprocedure to pre-stress a fabric-like hose or tube.

For many years research and development has been applied to retrofittingreinforced concrete by applying additional reinforcement(s). Thebeginnings of this technique are described in J. Bresson's "NouvellesRecherches Et Applications Concernant l'Utilisation des Collages DansLes Structures", Beton Plaque, Annales ITBTP 278 (1971), Serie Beton,Beton Arme Nr. 116. The description goes back to the sixties. Bresson'swork especially covered the requirements of compound stresses in thefield of anchoring bonded steel laminations.

Accordingly, it has been feasible for two decades to strengthen extantreinforced concrete structures, such as bridges, floor and paving slabs,longitudinal girders and the like, by subsequently bonding steellaminations.

The strengthening of concrete structures by bonding steel laminationsusing for example epoxy-resin adhesives, now may well be considered astandard technique.

Strengthening may be required for a number of reasons:

increasing the net load;

changing the static systems, for instance, removing post-facto bearingstructures such as uprights or struts or reducing their supportfunctions;

strengthening structures in danger of fatigue;

reducing the compliance;

damages in the bearing system or rebuilding extant structure; and

defective calculation or implementation of the structure.

Post-facto strengthening by means of bonded steel laminations has beenfound effective on numerous structures and, illustratively, is describedin the following literature: Ladner, M. & Weder, Ch., "GeklebteBewehrung Im Stahlbetonbau", EMPA Duendorf, Report No. 206 (1981);"Verstaerkung Von Tragkonstruktionen Mit Geklebter Armierung", Schweiz.Bauzeitung, Special print of 92nd year, Issue 19 (1974); "Die SanierungDer Gizenenbruecke Uuber Die Muota", Schweiz. Ingenieur & Architekt,Special print from Issue 41 (1980).

However, such strengthening procedures incur a number of drawbacks.Steel laminations can be delivered only in short lengths and, therefore,only short laminations can be used. Accordingly, the laminations mustbutt and potential weak spots must be incurred. The steel laminationsare heavy and their handling at the construction site may become quitedifficult if the pertinent structures are high or accessible only withdifficulty. As regards steel, even when carefully treated againstcorrosion, there is danger of the laminations under-rusting or thatthere will be corrosion at the interface of concrete and steel which maylead to detachment and hence to loss of the strengthening.

Accordingly, it was already suggested by U. Meier in "BrueckensanierungMit Hochleistungs-Faserverbundwerkstoffen", Material+Technik, 15th year,Issue 4 (1987) and further in H. P. Kaiser's dissertation DISS. ETH. No.8918, ETH ZURICH (1989) to replace the steel laminations by carbon-fiberreinforced epoxy-resin laminations. Laminations made of such materialevince low bulk density, high strength, excellent fatigue properties andoutstanding corrosion resistance. Accordingly, it is possible to usethin, lightweight carbon-fiber reinforced plastic laminations in lieu ofheavy steel ones. The plastic ones, furthermore, may be moved in anearly endless, rolled-up manner to the construction site. Practicaltests have shown that carbon-fiber laminations 0.5 mm thick evince atensile strength corresponding to the yield point of a 3 mm thick FE360steel lamination.

The present invention essentially starts from the ETH dissertation andin part represents a further development of the technical solutiondescribed therein for strengthening concrete structures. The contents ofthe ETH dissertation No. 8918 by H. P. Kaiser, Zurich ETH 1989 thereforeis an integral part of the present description and is further beingomitted.

The results of this ETH dissertation showed that the bending of concretestructures reinforced with carbon-fiber strengthened epoxy resins can becalculated similar to the way it is done for conventional steelreinforced concrete. However, special attention must be paid to theshear crack-formation in the concrete. Shearing cracks lead to offsetsat the strengthened surface and as a rule this entails peeling, i.e.,detachment of the reinforcing laminations. Accordingly, formation ofshear cracks is a significant criterion both as regards the bearingcapacity of the unstrengthened structure and any danger of detachment ofthe retrofitted strengthening lamina.

Accordingly, it is the object of the present invention to create amethod to strengthen a steel reinforced concrete structure or apre-stressed concrete structure against shearing forces to preclude asmuch as possible the occurrence of shearing cracks or at least toachieve a spectrum of finer cracks.

Another object of the invention is to so strengthen or protectstructures strengthened by strengthening laminations, preferably fibercompound laminations, against shearing forces in such a manner that theoccurrence of shearing cracks shall be extensively precluded at theinterface between the lamination and the concrete and to prevent as muchas possible any mismatch in the crack plane if such cracks do arise.

This object is achieved by the invention through a method defined in theclaim.

The invention directs that, in order to strengthen an elongated ortwo-dimensional bearing structure against shearing, pre-stressing means,which pre-stress substantially the cross-sectional area of thestructure, be mounted on the cross-section or in it. Advantageouslythese pre-stressing means are mounted essentially in the peripheral zoneor along at least part of the periphery of the structure's cross-sectionwhile pointing against this structure. The pre-stressing means can bemounted in a substantially slack or only slightly tensioned manneragainst the structure when the latter is only slightly shear-loaded ornot at all, as a result of which enhanced pre-stressing becomeseffective on or in the cross-section only at higher shear loads. On theother hand, the pre-stressing means already can be mounted in a highlytensioned manner against the structure when it is only slightly loadedor not at all.

Preferably, the above mentioned method of shear strengthening is appliedto those zones of the structure where shear forces may arise.

The invention furthermore involves a method to enhance the shearresistance of a construction structure with at least one elongated,lamellar reinforcement of the initially cited kind mounted externally onthe structure, the lamellar reinforcement being compressed at least inthose zones where shear forces arise by pre-stressing means transverseto the lamination and peripherally enclosing it against the structure.On account of such pre-stressing of the zones of the strengtheninglamination, the danger of shearing off the strengthening lamination issubstantially reduced in the face of shear forces. In addition, theforces applied by the lamination zones against the structure interface,i.e., against the concrete, so prevent or reduce the possibility ofshear cracks that in such an event the crack spectrum will contain finercracks.

In particular when using the fiber compound laminations proposed in theETH dissertation No. 8918, for example carbon-fiber laminations, it wasfound advantageous to prestress these laminations additionally mountedon the concrete structure to improve the structure's function and toprevent the lamination from shearing off on account of concreteshear-break in the tension zone. The high expansibility of thecarbon-fiber laminations amounts to a windfall as regards theaforementioned pre-stressing. The large elastic elongation and theYoung's modulus made to match the particular conditions favorably affectstress-losses due to shrinkage and creep. However, one aspect raisesproblems, namely anchoring the carbon-fiber laminations duringpre-stressing. The forces must be absorbed at least until full hardeningof the epoxy resin adhesive, for example by means of tensioningshackles.

Accordingly, the present invention further proposes a method forenhancing shear strength of an elongated or two-dimensional reinforcedconcrete structure fitted on the inside with a slack or pre-stressedsteel reinforcement. The elongated fiber compound lamination is mountedexternally to the structure being rigidly connected in the pre-stressedstate to the structure and the lamination is compressed at each end zoneagainst the structure by suitable pre-stressing means externallyenclosing this structure. On one hand, these pre-stressing means serveto anchor the lamination ends into the structure and, on the other hand,they ensure, by means of the pre-stressing forces directed against thestructure, that shear cracks cannot arise at the lamination ends,thereby substantially reducing the danger of the lamination shearingoff.

By mounting the above pre-stressing means, the critical factor no longeris shearing off the strengthening lamination but rather tearing it.However, because of the very high tensile strength of fiber compoundlaminations, this change represents substantial improvement.

Preferably, the pre-stressing means are lamellar or in the shape oftubes, hose-tubes, belts, bars or cables and are made of highlytear-resistant fabrics, illustratively, consisting of steel-, carbon-,glass- and/or aromatic poly-amide-fibers. However, otherfiber-reinforced plastics also are suitable as pre-stressing means, forinstance mono-axially, i.e., unidirectionally, stretched rovings or theabove cited fiber compound laminations suggested for strengthening. Thepre-stressing means applied on one side of the cross-section, or thepre-stressing means externally enclosing the lamination and directed atthe structure shall preferably be solidly anchored in the opposite zoneof the cross-section of the structure, for instance in the compressionzone, whereby pre-stressing is maintained. Preferably, the Young'smodulus and the geometry of the pre-stressing means shall be selected insuch a manner that drops in stressing on account of structure creep andrelaxation of the pre-stressing means shall be minimized.

If the structure comprises several inner shear reinforcements mountedessentially transversely to this structure, then the invention alsoproposes mounting or applying the pre-stressing means substantiallymidway between two inner shear reinforcements in or on one structurecross-section. When using fiber compound laminations, it can furthermorebe advantageous to mount, at intervals, pre-stressing means in the formof belts, bands, hoses, tubes or cables that are substantiallydistributed over the entire length of the lamination which they areexternally enclosing, the pre-stressing means forcing the laminationagainst the structure in order to counteract the detachment forcesacting on the lamination. In the case of extant inner shearreinforcements, it will be advantageous in this event also to mount thepre-stressing means each essentially midway between two inner shearreinforcements.

The above mentioned methods of the invention are especially applicableto shear-strengthening structures such as bridges, bearing or T beams,and floor or paving slabs. Basically, the methods of the invention aresuitable for shear-strengthening any construction structures, such asmade of steel reinforced concrete that serve as supports. The structuresfurthermore can be made also of other materials such as wood, metal,plastic and minerals other than concrete, etc.

Apparatus is described with which to carry out the method of theinvention and to enhance shear resistance on an elongated ortwo-dimensional bearing structure. The apparatus is characterized by atleast one stressing component in the form of a lamination, belt, hose,tube, band, bar or cable that is mounted in or on a cross-section of thestructure in essentially a slack or pre-stressed manner. Preferably, thestressing component is applied in nearly slack or pre-stressed manner atleast along a segment or at least against a portion of the periphery ofthe cross-section. Preferably, the stressing component consists of afabric or lattice material consisting of steel-, glass-, carbon- and/oraromatic polyamide-fibers or of another fiber-reinforced plastic such asunidirectionally stretched rovings or of the aforementioned fibercompound laminations suggested for strengthening.

A construction structure of the initially described kind, such as asteel reinforced concrete structure, can be strengthened using apparatusdefined in this invention against shear loads.

In particular, a construction structure with at least one externallymounted laminar reinforcement, such as a steel lamination or a fibercompound lamination can be further strengthened against shear using theapparatus of the invention. At least one stressing component is mountedin such a manner that it drives, i.e., compresses, the lamination whichit encloses externally transversely to its longitudinal directionagainst the structure. When using a fiber compound lamination, it shallbe mounted itself, preferably pre-stressed, on the structure.

The invention includes yet another method to pre-stress the abovepre-stressing means, more particularly a hose or a tube made of afabric-like material and the passing by at least one end through aborehole comprising a conical part flaring in the direction ofpre-stressing, and a viscous adhesive, for example a reactive glue whichis present in the conically flaring part. Thereupon, the tube is made topass in the pre-stressing direction through another borehole or a bushwhich in turn comprises a conical portion flaring in the pre-stressingdirection, and a wedge, i.e., a cone, essentially matching the conicalportion which is present inside the tube. The cone tip points in thedirection opposite that of pre-stressing. Lastly, pre-stressing isachieved by compressive or tensive means in such a way that the tube ispulled in the direction of pre-stressing through the first borehole andthrough the second borehole or bush and through the compressive ortensive means which preferably are rigidly affixed to the secondborehole or bush. The stress applied to the tube by the compressive ortensive means must be maintained until the above cited viscous adhesivehas substantially cured.

The described cone or wedge is roughened at least in parts of itssurface and, preferably, comprises at least one circular channeltransverse to the pre-stressing direction so that the wedge or coneshall be displaced into the tube when this tube is being pre-stressed bythe compressive or tensive means and generates a wedging effect so thatthe tube will be anchored.

The invention is elucidated below in an illustrative manner and inrelation to the attached drawings.

FIG. 1 is a lengthwise perspective of a concrete bearing beam comprisingthe shear strengthening means of the invention.

FIG. 1a is a section of the reinforced and/or stressed concrete beam ofFIG. 1 along line I--I of FIG. 1.

FIG. 2 is a longitudinal section of a reinforced-concrete bearing beamstrengthened by a fiber compound strengthening lamination.

FIG. 2a is a cross-section of the beam of FIG. 2.

FIG. 2b is a cutaway of the beam of FIG. 2 showing possible types ofrupture caused by shear loads.

FIG. 3 is a cross-section of the reinforced-concrete beam of FIG. 2fitted with shear-strengthening means of the invention.

FIG. 3a shows an end position of the beam of FIG. 2 in the rest zone andin the area of the additional fiber compound lamination, comprising twoshear-strengthening means of the invention.

FIGS. 4, 4a, 4b and 4c schematically show in longitudinal section themounting and pre-stressing of a strengthening lamination to a structureand the shear forces arising thereafter, and further the anchoring ofthe invention of the pre-stressed lamination into the structure.

FIG. 5 is a graph of beam sagging under load, for the unstrengthened,lamination-strengthened and pre-stressed-lamination strengthened states.

FIG. 6 is a slab structure comprising a strengthening lamination andshear-strengthening of the invention.

FIG. 6a is the slab structure of FIG. 6 shown in cross-section alongline II--II.

FIG. 7 is a schematic cross-section of apparatus and shows the principlewith which to pre-stress a hose or tube-like shear-strengthening meansat the structure to be strengthened and to anchor it.

FIG. 1 shows a schematic longitudinal section of a bearing beam 1, suchas a concrete or reinforced concrete beam. The shown concrete beamcomprises longitudinal steel reinforcement 7 to impart higher bearingcapacity to the beam under load.

In order to oppose shearing cracks when the bearing beam 1 is undershear stress, i.e., to strengthen the bearing beam against shearingforces, the invention provides pre-stressing means 11 in each of thebeam cross-sections 4. These pre-stressing means either lie slacklyagainst the outer contour 15 in the cross-section 4 or they are mountedcompressively against it. Furthermore, they are rigidly anchored intothe structure 1 at the sites 13. If they are slack, the pre-stressingmeans 11 will be tightened only upon shears being applied to the bearingbeam 1.

FIG. 1a shows a cross-section 4 along line I--I of FIG. 1. Thepre-stressing means 11 shown in FIG. 1a can be, for example, a highlytear-resistant, well-stressing fabric or mono-axially stretched rovingsin the shape of a cable, belt, hose, tube, lamination, bar or band andruns on one hand through the two boreholes 6 in the structure and on theother hand encloses the periphery of the cross-section 4 along thesegment 15. The pre-stressing means 11 either rest substantially slacklyagainst the segment or else compressively pre-stressing it. To achievebetter distribution of the stressing force on the segment 15 and on theother hand to preclude excessively loading the stressing means 11 onboth sides of the segment 15 at the exit of the boreholes 6,advantageously a substrate 16 is provided which, illustratively, canconsist of fiber compound materials. Obviously, steel or any othermaterial can also be used, the point being that a stress applied orgenerated in the stressing means 11, such as a hose, tube, cable etc.,shall be maintained. It is important therefore that the pre-stressingmeans 11 be anchored into the sites 13 in a problem-free manner.

FIG. 2 shows a longitudinal section of a steel reinforced concrete beam1 resting by its ends in zones 2 and 3 on supports 5. The concrete beamalso comprises a steel reinforcement 7 and shear reinforcements 8transverse thereto. In the sense of the initially discussed ETHdissertation No. 8918, the bearing beam 1 is further fitted with astrengthening lamination 21, illustratively made of a carbon fibercompound material using an epoxy resin matrix. FIG. 2a shows the bearingbeam 1 of FIG. 2 in cross-section and makes plain that it is a T beam.The strengthening lamination 21 employed can be steel or it can consistof any fiber compound material such as described in the cited ETHdissertation No. 8918. The dissertation is referred to for a descriptionof the advantages in using fiber compound laminations as well as oftheir shapes, sizes and how to mount them on the structure, andtherefore this discussion is omitted herein.

Now it has been found that in the event of exceedingly high shears,various kinds of breaks can arise even in a structure strengthened withsuch an additional lamination. Various possible kinds of breaks areschematically shown in the cutaway of the bearing beam 1 of FIG. 2 inFIG. 2b. The break type referenced by 31 is concrete upset in thecompression zone, reference 32 is a steel break in the tensive zone, 33is a lamination break, 34 is a cohesion rupture at the concrete surface,35 is an adhesion rupture at the lamination surface, 36 is an adhesionrupture at the concrete surface, 37 is an inter-laminar break of thelamination and reference 38 denotes a concrete shear-rupture in thetensive zone which, as a rule, leads to shearing the lamination 21 offthe beam 1.

In order to oppose in particular the concrete shear-rupture in thetensive zone, but also the other kinds of breaks, foremost at theinterface between the lamination and the concrete beam 1, the inventionpresents a shear strengthening means as shown in FIG. 3 in the form ofthe pre-stressing means 11 described in relation to FIG. 1. FIG. 3 againshows the bearing beam of FIG. 2 in cross-section, however it is nowfitted with a pre-stressing means 11 also again anchored in sites 13 ofthe concrete beam 1. The pre-stressing means 11, illustratively, is anaramide-fiber tube and passes on both sides through boreholes 6 in theupper slab of the bearing beam 1 and then through both sides along thebase body of the bearing beam and then encloses in its pre-stressedstate the strengthening lamination 21 at the lower end of the beam.Again a substrate 16 is provided to make possible improvedstressing-distribution by the aramide tube 11 against the lamination 21and furthermore to preclude damage to the tube 21 in the region where itloops the base body of the bearing beam 1 and the lamination 21. Ideallythe substrate 16 would be semi-circular to achieve optimal compressiondistribution. However, adequate distribution is already achieved by thesubstrate 16 shown in FIG. 3 which assuredly shall be more advantageousin practice.

As already described, the substrate 16 must be such that the stress inthe aramide tube 11 shall be kept up rather than being lessened byforcing the tube 11 into the substrate 16 and/or by compressing thesubstrate 16.

FIG. 3a shows the beam-end zone 2, similarly to FIG. 2, in the area ofthe support 5. FIG. 3a makes it clear that the pre-stressing means 11are advantageously mounted in the end zone of the lamination 21 becausethat is where the danger of shearing off the beam 1 is largest. Suchshearing off results not on account of inadequate adhesion of thebonding layer 20, but especially by the concrete compressive breaks inthe structure shown in FIG. 2b.

As shown by FIG. 3a, it has been found advantageous to mount at leasttwo pre-stressing means 11, such as aramide tubes, in the end zone ofthe lamination 21. Where possible, the additional pre-stressing means 11of the invention are mounted essentially mid-way in the area of twotransverse shear reinforcements 8 of the structure 1. The primaryimportance, however, is to optimally press the end of the pre-stressedlamination 21 against the structure 1.

If on the other hand further pre-stressing means 11 of the invention areprovided over the entire length of the lamination 21, namely to preventthis lamination from detaching anywhere from the beam, thenadvantageously the illustrative aramide tubes are mounted essentiallymid way between two shear reinforcements 8.

For shearing forces arising especially in the two end zones 2 and 3 ofthe loaded bearing beam 1, the shear strengthening means of theinvention advantageously will be mounted especially in these two endzones. In principle, the shear strengthening means of the inventionassume a function similar to the shear reinforcements inside thestructure which, as shown by FIG. 2, also are preferably located in thetwo end zones 2 and 3 of the bearing beam 1.

As already mentioned, the strengthening lamination 21 can beadvantageously pre-stressed. This is especially appropriate when usingfiber compound laminations on the basis noted above.

The technique of pre-stressing such laminations is schematically shownin FIGS. 4, 4a and 4b.

FIG. 4 is a longitudinal section of a bearing beam 1 near the end zone 2which is to receive a pre-stressed fiber compound lamination 21.

As shown by FIG. 4a, the lamination 21 is stressed in the direction ofthe tip of the end zone 2 of the beam 1 by applying a force P₀. Whilebeing pre-stressed, it is firmly connected by depositing an adhesivelayer 20, for example an epoxy resin, on the bearing beam 1. Thelamination 21 can be pre-stressed with an entirely arbitrary tensioningor stressing apparatus. Such a procedure for pre-stressing is generallyknown and, in particular, is described in the ETH dissertation No. 8918and therefore it is omitted herein.

Now FIG. 4b shows what happens, indicated by Δx, in the end zone of thebeam 1 in the absence of the tensive force P₀. The stress in thelamination 21 generates the shearing stress S in the structure, as aresult of which there is danger of shear cracks arising in the area 2aof the beam 1. If the cracks were to grow to a certain size, thelamination consequently would shear off in impulsive manner and as arule detachment would propagate toward the beam center. Thereby thedesired strengthening of the beam would be lost.

FIG. 4c shows the shear strengthening means of the invention mounted inthe end zone of the lamination 21, a force F acting on the lamination 21in the direction of the beam 1. Thereby the formation of cracks shall beminimized by a multi-axial stress in the concrete. When cracks occur,serrating them effectively allows further successful anchoring of thelamination into the structure. In the manner of FIG. 3a, two aramidetubes 11 are mounted in FIG. 4c and are pre-stressed over a substrate 16against the end zone of the lamination 21. The lamination 21 is anchoredby pre-stressing means 11 in the same manner at the opposite but omittedend of the beam 1 into it.

The graph of FIG. 5 shows the advantageous effect of pre-stressing thelamination 21 on the loading capacity of a bearing beam. Areinforced-concrete beam similar to the one shown in FIG. 2 is proppedup and increasingly loaded and the corresponding sagging observed. Line25 of the graph of FIG. 5 shows the reinforced-concrete beam without anexternal lamination strengthening, line 26 shows the same beam nowprovided with a carbon-fiber lamination, and line 27 shows again thesame beam fitted with the same carbon-fiber lamination pre-stressed forinstance between 0 and 90% of its tensile strength and being anchored ateach end zone with pre-stressing means of the invention into the bearingbeam. Line 27 shows the largest load-bearing capacity for the bearingbeam by the pre-stressed carbon-fiber lamination.

When pre-stressing in the above manner by a magnitude exceeding about 5%the tensile strength of the lamination, use of the pre-stressing meansof the invention, such as the aramide-fiber tubes, will be mandatorybecause otherwise the laminations shall be immediately sheared off theend zones. Tests have shown that carbon-fiber laminations can be mountedon a bearing beam only for a stress up to 50N/mm² before thepre-stressing means of the invention become necessary. Higherpre-stressing forces at once caused lamination detachment.

In order to reliably anchor a lamination of FIG. 4c into a bearing beamwhen the pre-stressing forces are approximately the above mentionedmagnitudes, the aramide tubes are endowed with a tensile strength pertube of 25 kN.

In order to maintain such high tensile strengths in the pre-stressingmeans, for instance aramide tubes, it is obviously mandatory that theybe reliably and solidly anchored into the concrete support at the zonesopposite the strengthening lamination.

A method for effectively anchoring such tubes is discussed further belowin relation to FIG. 7.

First, it will be shown in relation to FIGS. 6 and 6a how theshear-strengthening means of the invention can be anchored in similarmanner into a concrete slab. FIG. 6, similar to FIG. 1, is a lengthwiseperspective of a concrete slab 1. The pre-stressing means of theinvention is mounted in the manner of the invention in the cross-section4 and is solidly anchored into sites 13 of the concrete slab or paving.Furthermore, the concrete paving or floor slab 1 comprises at itsunderside an elongated carbon-fiber strengthening lamination 21 which issimilar to those discussed above.

FIG. 6a shows the cross-section along line II--II of FIG. 6 andcorresponds substantially to FIG. 1a. The pre-stressing means, i.e., theshear strengthening means 11, runs from the anchoring sites 13 throughboreholes 6 in the concrete slab to the opposite side of thecross-section and encloses a compression plate 16 pressing against thelamination 21. The lamination 21 in turn rests against the segment 15 atthe periphery of the cross-section 4. Because of the pre-stressing ofthe means 11 which, illustratively, is a fabric-like belt or band, thecompression plate 16 is forced against the lamination 21 and thereby thelamination 21 is prevented from shearing off the concrete paving or slab1 in the vicinity of the segment 15 of the cross-section 4. Obviously,the concrete paving or slab 1 can be additionally fitted with steelreinforcements as shown for example in FIG. 2 and following.

The concrete structures shown in FIGS. 1 through 6 are merelyillustrations serving to elucidate the invention. Obviously suchstructures also can be bridges, pavings, floor slabs,reinforced-concrete beams or any other two-dimensional or elongatedconstruction structure, including those covering several surfaces andmade of very diverse materials such as wood, metal, concrete etc. whichmust serve as supports. Again the manner of pre-stressing of theinvention in or at a cross-section of such a concrete structure can beimplemented in entirely arbitrary manner. Pre-stressing can be appliedto the structure not yet loaded or only slightly loaded, or thepre-stressing means can be applied while being very slack or onlyslightly tensioned so that increased pre-stressing only takes place atincreased loading, i.e., shearing of the structure. Obviously too theshear-strengthening methods and means of the invention can be used in anew building or in restoring an extant one. The choice of thepre-stressing means as well is manifold, and in lieu of theabove-described, specifically designed fabric materials, so-calledunidirectionally stretched rovings or carbon-fiber laminations can beused which are similar to those shown in the Figures and denoted as 21.However, steel bands, cables, belts and the like made of other materialsevincing high strength are applicable in the invention.

Accordingly, the concept of the invention can be modified in many ways,it being essential that by selecting the pre-stressing means in or at across-section in the concrete structure to be strengthened there shallbe achieved at least segment-wise pre-stressing to effectivelycounteract the shear forces arising upon loading.

Lastly, FIG. 7 shows by a schematic cross-section how, for instance anaramide-fabric tube, can be pre-stressed and anchored in the structure.The aramide tube 11 is pulled by tensive, stressing or compressive means(not shown) in the direction of the arrow 50, at first through theborehole 6 in the structure 1. A conical widening 41 is present in thestructure 1 at the site 13 and in this conical widening the tube 11 isexpanded in the zone 42 by placing an adhesive 43 inside the zone 42. Onaccount of gravity, the highly viscous adhesive 43 flows in thedirection of the arrow toward the borehole 6. The adhesive, for examplecan be an epoxy resin or a thermoplastic polymer melt.

Thereupon, the aramide tube is pulled through a circular bush 44 mountedon the structure 1 and the inside of the bush again is conical flaringin the longitudinal direction. By mounting a wedge or cone 45 inside thetube, the tube once more is widened inside the bush. The cone 45preferably is roughened at its surface and comprises furthermoretransverse annular recesses 46 to allow a follow-up slippage of thewedge when the tube 11 is pulled in the direction of the arrow 50 and toachieve an immediate wedging effect once the force 50 drops. To preventthe bush 44 from moving back toward the structure, it can be affixed,for example, by a thread 47 to a casing 48.

Finally, the tube 11 is pulled by its segment 49 through a tensive,stressing or compressive means (not shown) in the direction of the arrow50 until adequate tension has been achieved. This tension is maintaineduntil the adhesive 43 has completely and adequately cured. Depending onthe choice of adhesive, this can amount to a few minutes or severalhours.

The advantage offered by the pre-stressing means shown in FIG. 7, i.e.,the anchoring of the pre-stressing means into the structure 1, is theelimination of additional mechanical anchors. Moreover, accuratepre-stressing can be provided and this pre-stressing level shall besubstantially preserved following the anchoring of the pre-stressingmeans 11. Lastly, the pre-stressing means 11 can be finished to be flushwith the surface of the structure 1 so as to eliminate any projections.

The diagrammatically shown pre-stressing method of FIG. 7 is suitablefor any tubular pre-stressing means such as the above noted aramidetubes. Obviously there is no compulsion that the tube be fabric-like andthe materials employed can be selected in a wholly arbitrary manner.Obviously the advantage of material selection is that the widening inthe zone 42 holding the adhesive inside the tube is substantiallysimpler and better than if, for example, a substantially "solid" tubewere used.

There is ample selection from the available materials, illustrativelythese being steel-, glass-, carbon-fibers or others. The essential pointis that a tube of high tensile strength can be formed.

We claim:
 1. A method for increasing the shear strength of an elongatedbearing member having a length and a width comprising mounting at leastone pre-stressing means in relation to at least one cross-sectional areaof said elongated bearing member so that said at least one pre-stressingmeans is positioned essentially transverse to the length of saidelongated bearing member in a pre-stressed manner, and further in saidmounting of said at least one pre-stressing means, positioning saidpre-stressing means in a pre-stressed manner having ends embedded atleast in part in the elongated bearing member and having an intermediateportion positioned at least in part along a peripheral surface of the atleast one cross-sectional area of said elongated bearing member.
 2. Amethod according to claim 1 wherein said mounting of said at least onepre-stressing means comprises mounting two pre-stressing means inrelation to two separate cross-sectional areas of said elongated bearingmember, one pre-stressing means in each of said cross-sectional areas.3. A method according to claim 1 wherein said elongated bearing memberincludes at least one longitudinal lamellar reinforcement externallymounted thereon, and further comprising in said mounting of said atleast one pre-stressing means, compressing of said at least onelongitudinal lamellar reinforcement by said at least one pre-stressingmeans.
 4. A method according to claim 1 wherein said at least onepre-stressing means is of a form selected from the group consisting of alamination, tube, hose, belt, band and cable, and made of atear-resistant material selected from a group consisting of steelfibers, carbon fibers, glass fibers, aromatic polyamide fibers,fiber-reinforced plastics, and fiber compound laminations.
 5. A methodaccording to claim 1 wherein said elongated bearing member includes aplurality of inner shear reinforcement elements positioned transverselytherein, and wherein in said mounting of said at least one pre-stressingmeans, said at least one pre-stressing means is mounted substantiallymidway between two of said plurality of inner shear reinforcementelements.
 6. A method according to claim 1 wherein said elongatedbearing member is a member of a bridge, bearing beam, T-beam, floor slabor paving slab.
 7. A method for increasing the shear strength of anelongated bearing member having a length and a width comprising mountingat least one pre-stressing means in relation to at least onecross-sectional area of said elongated bearing member so that said atleast one pre-stressing means is positioned essentially transverse tothe length of said elongated bearing member in a pre-stressed manner,and further in said mounting of said at least one pre-stressing means,positioning said pre-stressing means in a pre-stressed manner at leastin part along a peripheral surface of the at least one cross-sectionalarea of said elongated bearing member, and further comprising affixingexternally at least one pre-stressed elongated fiber lamination to saidelongated bearing member such that said at least one lamination iscompressed at each end thereof against said elongated bearing member bysaid at least one pre-stressing means.
 8. A method according to claim 7further comprising mounting said at least one pre-stressing means toexternally enclose a portion of said at least one pre-stressed elongatedfiber lamination so as to pre-stress said lamination at intervals and inturn compress said elongated bearing member.
 9. An elongated bearingmember having a length and a width, said elongated bearing member havingincreased shear strength and comprising at least one pre-stressing meanshaving a form selected from a group consisting of a lamination, belt,hose, tube, band and cable, wherein said at least one pre-stressingmeans is present in a pre-stressed manner in at least onecross-sectional area of said elongated bearing member, extendssubstantially transverse to the length of said elongated bearing member,and has ends embedded at least in part in the elongated bearing memberand has an intermediate portion which rests against at least a portionof a peripheral surface of said at least one cross-sectional area. 10.Elongated bearing member according to claim 9 wherein said at least onepre-stressing means is made of a material selected from a groupconsisting of steel fibers, glass fibers, carbon fibers and aromaticpolyamide fibers.
 11. Elongated bearing member according to claim 9wherein said elongated bearing member is a member of a bridge structure.12. Elongated bearing member according to claim 9 further comprising atleast one lamellar reinforcement externally mounted on said elongatedbearing member with said at least one pre-stressing means mounted onsaid elongated bearing member so as to compress said at least onelamellar reinforcement transversely against said elongated bearingmember.